Separatory membrane fabrication currently demands the preparation of microporous separatory membranes having such exacting specifications as ultra-fine pore size with a lower limit of about 10 A, resistance to degradation by chemicals, pressure and heat, and specific surface characteristics of hydrophobicity and hydrophilicity. The search to find such specific materials for each individual separatory purpose has often led to expensive and complicated fabrication procedures.
According to the investigations of this invention, by using hydrophobic microporous membranes as the base substrate, plasma polymerization deposition techniques can create novel ultrafiltration membranes which satisfy such precise requirements. Usually, hydrophobic microporous membranes, of the type used as the base substrate for preparing the novel ultrafiltration membranes of this invention, have large pores, in the range of at least about 200 A to at most about 6000 A. While these currently available membranes have a high resistance to chemicals, heat and pressure, due to their simpler polymeric structure, as compared to conventional ultrafiltration membranes, they are not suitable for ultrafiltraton purposes because of molecular weight cutoff due to their unacceptably high pore size.
By controlling the deposition of a plasma polymer, the pore size of these hydrophobic microporous membranes can be reduced from their original size to any specifically required smaller size, down to a lower limit in the range of about 10 A. Because the plasma polymer deposited layer is extremely thin (in the range of 1200 A or less), the membrance substrate retains its porous character, without any significant decrease in the permeabilities of liquids and gases, but with an increased ability to separate ultra-fine particulate from solution.
Conventional plasma polymerization coating techniques, as applied to the fabrication of composite membranes, have all suffered from certain inherent disadvantages, regardless of the type of reactor systems utilized (i.e., Bell Jar reactors, A.F. tandem systems, R.F. coil-inductively coupled tubular reactors). These disadvantages, generally, are due to the fact that such conventional plasma polymerizations involved deposition of the polymer onto a substrate in situations where the energy density of the plasma, and thus the uniformity of the polymer deposition, could not be easily controlled. These disadvantages can generally be summarized as follows:
1. Non-uniformity in plasma polymer deposition rates and plasma polymer coating composition, primarily dependent on the substrate's position in the reactor.
2. Low deposition rates due to low energy density levels leading to very slow membrane production rates. Low energy density levels are encountered with conventional plasma reactors, whether of the Bell Jar or R.F. coil-inductively coupled tubular type, where polymer deposition takes place in the "after glow" zone, or of the A.F. type, where polymer deposition takes place in the glow zone. Low deposition rates in conventional reactors can further be attributed to the build up of plasma coating of the internal electrodes.
3. Inability to evenly and effectively coat multiple membrane substrates, due to competitive shading from the plasma glow, and due to the fact that polymer deposition rates are primarily dependent on the precise position of the substrate in the reactor.
4. Non-uniformity in coating around the exterior of the membrane substrate, for example, around the circumference of a fiber.
5. Problems in the undesirable formation of multiple chemical species, and the inability of efficiently remove waste chemical species.
6. Difficulty in controlling all of these plasma deposition reaction parameters, particularly in scale-ups to commercial production.
This inventor has now unexpectedly discovered that the use of an R.F. capacitively coupled tubular reactor, specifically controlled in terms of system operation, results in plasma polymer deposition techniques, that are highly reliable, able to operate at high production rates, and produce a highly desirable uniform product. According to the process of this invention, the plasma polymer is deposited on the microporous membrane substrate moving through the energy-intensive glow zone in the region between the electrodes of the reactor. It is well known in the art that microporous membrane substrates are extremely difficult to plasma coat due to their sensitivity to manipulative stresses, such as temperature, pressure, tension, and chemical attack. However, this inventor now unexpectedly discloses that microporous membrane substrates can be plasma coated in this manner with speed and efficiency and with uniform desirable results.